Abstract
The solubilization of aromatic compounds (phenol, benzoic acid, benzene, ethylbenzene) in the aqueous solutions of didodecyldimethylammonium bromide (DDAB) vesicles was investigated as a function of temperature by measuring the solubilization equilibrium constants (Kvesicle), the microviscosity of the vesicle bilayer membrane, and the degree of the dissociation (α) of the counter-ion from the vesicle. Benzene and ethylbenzene are only sligthtly solubilized below the temperature of 8°C, and Kvesicle between the DDAB vesicle and the bulk aqueous phase shows an abrupt increase at the temperature range 810°C. The fluorescence measurement indicates that the phase transition temperature (Tc) of the DDAB vesicle is about 9.5°C. The solubilization of benzene and ethylbenzene will depend on the microviscosity of the vesicle bilayer membrane. The comparison of Kvesicle with the partition coefficient (Kdodecane) of the aromatic solute between the n-dodecane phase and the water phase suggests that the benzene molecule is solubilized at the vesicle surface where the π electron cloud of the benzene interacts with the changed surfactant head groups, and that the ethylbenzene molecule is located inside the bilayer membrane. On the other hand, Kvesicle values of phenol and benzoic acid exhibit a maxima at ca. Tc of DDAB, and the α value of the vesicle also has a maximum around Tc. It is thought that the solubilization of phenol and benzoic acid depends on the degree of the DDAB vesicle charge, and these polar solutes are located at the vesicle surface because the polar groups are attracted electrostatically to the charged head groups of DDAB.
